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Date: Tue, 15 Sep 2015 07:09:42 -0500
From: Jeff Epler <jepler@unpythonic.net>
To: Tejun Heo <tj@kernel.org>
Cc: John Stultz <john.stultz@linaro.org>, LKML <linux-kernel@vger.kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Ingo Molnar <mingo@kernel.org>,
        "Steven Rostedt (Red Hat)" <rostedt@goodmis.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>,
        Michal Nazarewicz <mina86@mina86.com>,
        Prarit Bhargava <prarit@redhat.com>,
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        Thomas Gleixner <tglx@linutronix.de>, "Theodore Ts'o" <tytso@mit.edu>,
        Andreas Dilger <adilger.kernel@dilger.ca>,
        Dave Chinner <dchinner@redhat.com>, Joe Perches <joe@perches.com>
Subject: Re: [RFC][PATCH 0/5] Fixes for abs() usage on 64bit values
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On Mon, Sep 14, 2015 at 11:46:32PM -0400, Tejun Heo wrote:
> Hello,
> 
> On Mon, Sep 14, 2015 at 08:27:08PM -0700, John Stultz wrote:
> > Yea. The above make sense to me, but I suspect there's some very
> > subtle reason for the existing separated logic.
> > But I'd have to defer to akpm for hints on that.
> 
> Hmmm... people could be using it for calculating the distance between
> two unsigned values and in that case the original behavior would be
> the correct one.  e.g.
> 
>  unsigned diff, a, b;
>  diff = abs(a - b);

This kind of construct can overflow whether a and b are signed or
unsigned.  Only a two-argument function can correctly return the
absolute difference of two integers.

At my day job, we arranged for
abs(unsigned type) to be a compile-time error and supplied a two args
function absdiff for use in these situations -- absdiff(a,b) is the same
as abs(a-b) except it avoids overflow and returns an unsigned type.

(though I have no doubt that some instances of abs(a - b) still exist
where a and b are signed and the intermediate could still overflow...)

Jeff